Project Summary While evidence exists for the efficacy of particular substance use policies on the targeted substance, little attention has been given to cross-substance policy effects ? i.e. the effect of a policy targeting one substance on the use of another substance. This study will be the first to establish robust empirical evidence for cross- substance policy effects using the case of tobacco and marijuana. With marijuana or tobacco in wide use among youth, policies that target either substance may residually affect use of the other. Both singular and co- use of tobacco and marijuana are associated with numerous adverse health effects. Though macro-level and repeated cross-sectional studies of policies and substance use provide some evidence of policy effectiveness on targeted substances, little research has examined how policies at various levels implementation (e.g. local, county, and state) affect within-individual trajectories (initiation, prevalence, duration, and cessation) of tobacco and marijuana use during the critical teenage and young adult years, and none consider cross-substance policy effects. With an approach that examines the cross-substance influence of policies simultaneously on the same individuals over time, policymakers can more adequately assess substance control policies and, thus, invest in highly efficacious regulations that impact substance use widely. Using nationally representative longitudinal data, a complete record of tobacco control ordinances and newly collected data on marijuana policies, and innovative statistical methods, the goal of this project is to develop an understanding of the influence of cross-substance policy effects, which have not been effectively studied to date. Importantly, we will also assess the mechanisms of these influences by examining the direct and indirect effects of policy. To fully understand how cross-substance policy effects influence youth marijuana and tobacco use patterns over time, we require a geocoded, repeated observations dataset of youth substance use and predictive risk factors, a comprehensive database of regulations, and a statistical method capable of handling a complex hierarchical data structure. We address those needs by merging data from the annually collected National Longitudinal Survey of Youth 1997 with the Americans for Nonsmokers' Rights national repository of tobacco regulations and a newly collected geocoded dataset of marijuana laws. We will merge these geocoded datasets and analyze them with hierarchical generalized linear models and cross-lagged panel models. After controlling for well-known risk factors and local structural conditions, the efficacy of particular policies (as well as their direct and indirect pathways) will be demonstrated, ultimately impacting health and mortality and reducing costs to society. This proposal's innovation lies in being the first empirical examination of cross-substance policy effects. Additionally, by using the case of within-youth changes in marijuana and tobacco use over time, we will provide innovative evidence of policy influences on an important public health problem among young people.